Golf club head manufacturers and designers are constantly looking for ways to improve golf club head performance, which includes the forgiveness and playability of the golf club head, while having an aesthetic appearance. Generally, “forgiveness” can be defined as the ability of a golf club head to compensate for mishits, i.e., hits resulting from striking the golf ball at a less than an ideal impact location on the golf club head. Similarly, “playability” can be defined generally as the ease in which a golfer having any of various skill levels can use the golf club head for producing quality golf shots.
Golf club head performance can be directly affected by the moments of inertia of the club head. A moment of inertia is the measure of a club head's resistance to twisting upon impact with a golf ball. Generally, the higher the moments of inertia of a golf club head, the less the golf club head twists at impact with a golf ball, particularly during “off-center” impacts with a golf ball. The less a golf club head twists, the greater the forgiveness of the golf club head and the greater the probability of hitting a straight golf shot. In some instances, a golf club head with high moments of inertia may also result in an increased ball speed upon impact with the golf club head, which generally translates into increased golf shot distance.
In general, the moment of inertia of a mass about a given axis is proportional to the square of the distance of the mass away from the axis. In other words, the greater the distance of a mass away from a given axis, the greater the moment of inertia of the mass about the given axis. Accordingly, golf club head designers and manufacturers have sought to increase the moment of inertia about one or more golf club head axes, which are typically axes extending through the golf club head center of gravity, by increasing the distance of the head mass away from the axes of interest.
United States Golf Association (USGA) regulations and constraints on golf club head shapes, sizes and other characteristics tend to limit the moments of inertia achievable by a golf club head. According to the most recent version of the USGA regulations, golf club heads must, inter alia, be generally plain in shape, have a reasonable and traditional head mass between 203 and 213 grams, have envelope dimensions at or below maximum envelope dimensions (maximum height of 2.8 inches, maximum width of 5.0 inches and a maximum depth of 5.0 inches), and have a volume at or below a maximum head volume of 460 cm3. It should be noted that this maximum volume constraint of 460 cm3 is well below the volume of the maximum envelope dimensions.
Often, golf club manufacturers are faced with the choice of increasing one performance characteristic at the expense of another. For example, the shape and size of some conventional golf club heads approach the maximum envelope dimensions in an attempt to increase the moments of inertia of the heads. Such designs, however, most likely require a decrease in the face size, or ball striking surface area, in order to comply with the USGA regulations. As another example, some conventional golf club heads have an increased face size in an attempt to optimize the ball striking surface of the golf club head. Such golf club head designs, however, typically have decreased moments of inertia.
Golf club designers and manufacturers have struggled to design golf club heads having increased moments of inertia while maintaining other desirable golf club head characteristics and abiding by the USGA regulations.